This invention relates to communications systems, data transmission, modulation and coding, turbo codes, a bandwidth efficient advanced modulation waveform, and more specifically a carrier and time tracking algorithm for use in these systems.
In digital communications systems such as cellular and PCS (personal communications systems), computer communications systems, and SATCOM (satellite communications) systems digital data is modulated by a modem onto a signal to be transmitted over a communications channel. The communications channel may add noise, interference, fading, and other corrupting influences that may result in loss of the data when demodulated at a receiver in the communications link. Channel coding has been used in communications systems for many years to detect and/or correct data bit errors by introducing redundant bits. The use of channel coding results in a reduction of data rate or an increase in required bandwidth due to the additional redundant bits.
Block codes and convolutional codes are two types of channel codes commonly used in the art of channel coding. A block code is an error detection and/or correction code in which an encoded block of data consists of n coded bits, containing k information bits (k<n) and n−k redundant check bits to detect and/or correct most errors. Types of block codes known in the art include Hamming codes, Golay code, BCH codes, and Reed Solomon codes.
Convolutional codes are widely used in the communications art to provide error correction. Convolutional codes continuously convert an entire data stream to encode the k information bits. The encoded bit stream depends on the current information bits and also on the previous input information bits. With a convolutional code, k information bits are coded into n coded bits in an encoder with m memory stages that store the state information of the encoder. A constraint length K of a convolutional code is defined as m+1 and a code rate r as k/n. The well-known Viterbi algorithm is commonly used to decode convolutional codes.
Recent advances in the art of coding to further improve error detection and correction while reducing bandwidth requirements include trellis code modulation. Trellis code modulation combines coding and modulation into one operation. By combining coding and modulation, redundancy can be obtained with no reduction in data rate or increase in bandwidth.
Continuous phase modulation (CPM) is being applied in communications due to its bandwidth efficiency and constant envelope characteristics. With CPM, the modulated signal phase transitions are smoothed. With BPSK (binary phase shift keying) a logic one is transmitted as one phase of a modulated signal and a logic zero is transmitted as a 180-degree shifted phase with a sharp transition in phase. This sharp phase transition results in broadening of the transmitted spectrum. With CPM the phase of the transmitted signal makes smooth phase changes over the bits of the modulating digital signal. An example of CPM currently in use is MSK (minimal shift keying) modulation.
Turbo codes allow reliable transmission of data across a communications channel near the theoretical limit predicted by Claude Shannon. A turbo code is generated at a transmitter by a serial or parallel concatenation of two or more component codes often recursive convolutional codes, each separated by an interleaver. For the common case of a two constituent code turbo code, turbo decoding at a receiver uses a soft decoder at the input followed by an inverse interleaver and a second soft decoder. The output of the second soft decoder feeds back to the input of the first soft decoder through an interleaver. The data is passed through the turbo decoder in several iterations with each pass improving the quality of error correction.
A new data communications waveform has been developed by Rockwell-Collins called BEAM (bandwidth efficient advanced modulation). The goal of a current application of the BEAM waveform is to increase the throughput of a typical 25-kHz UHF SATCOM channel by a factor of five over the MIL-STD-188-181A PSK (phase shift keying) waveform, while maintaining a reasonable Eb/No. The throughput would thus increase from the current standard data rate of 16 kbps to 80 kbps. The BEAM waveform must operate with current UHF satellites and thus must have a constant envelope. The constant envelope requirement is important for this application because all current UHF satellites use saturating amplifiers. In addition, most, if not all, of the UHF user terminals utilize saturating amplifiers for power efficiency. The BEAM waveform jointly combines coded CPM modulation with turbo coding. The joint combination provides superior performance over that which would have resulted by simply concatenating CPM with turbo coding. Although the BEAM waveform has been invented for UHF SATCOM applications, the BEAM waveform concept can be extended to almost any frequency band and almost any communications channel. Because of the underlying CPM modulation, it is particularly useful to those communications systems that rely on saturated amplifiers and suffer from the effects of AWGN (additive white Gaussian noise).
The recent developments in modulation and coding theory, such as turbo codes, have allowed communication capacity to move much closer to Shannon's information limit. While these developments are greatly welcomed, the issue of synchronization has received much less attention. Synchronization for modems consists of two parts. The first type of synchronization is carrier synchronization that requires that a replica of the original carrier waveform be reproduced. The second type is symbol timing synchronization that specifies where the center and thus the optimum sampling point of each symbol are located. When turbo codes are jointly combined with modern modulation methods, the large delay introduced by the turbo code interleaver/deinterleaver prevents traditional methods of decision-directed feedback from being applied to the problem of synchronization. If decision-directed feedback is used in the conventional way, the large delay causes very poor synchronization loop tracking performance.
What is needed is a method and apparatus for achieving and maintaining carrier and timing synchronization and tracking in a low signal-to-noise ratio (SNR) environment for modulation/coding systems that make use of a turbo code. The specific problem to be solved is how to obtain reliable symbol decisions and how to use those decisions in a decision-directed feedback loop.